Field of the Invention
The present invention relates to hollow waveguide sensors, and more specifically, it relates to substrate-Integrated hollow waveguide sensors.
Description of Related Art
Conventional hollow waveguides as they are known by those knowledgeable in the field were first developed as a conduit for transmitting laser light for industrial and biomedical applications. Those applications relied on hollow waveguide length (i.e., often meters), flexibility, and a high damage threshold. In recent years as their use has extended to include spectroscopic applications, the adopted conventional hollow waveguides are not suitable for integration into sensing devices because of two key characteristics, length and flexibility. For example in high-sensitivity infrared (IR) gas sensing applications involving hollow waveguide sensors, the prior art requires a large operational footprint because the conventional hollow waveguides must be physically long (typically 0.5 to 4 meters in length of drawn tubular glass having an optical coating at the inside of the hollow core), because physical length is needed to increase the sampling path length to obtain trace levels of detection for gas analytes. The flexible conventional hollow waveguide must also be supported over these lengths and in a manner that ideally maintains a constant temperature and eliminates vibrations, either of which can alter the light guiding modes of the waveguide.
In the prior art, the dimensions of the waveguide largely govern the size of the overall gas sensor, thereby limiting its practical use for many applications. In the prior art, coiling the waveguide has been demonstrated to reduce its form factor, however, the results show substantial signal attenuation due to bending losses (attenuation coefficient varies as 1/R, where R is the bending radius). Furthermore, there are no long-term studies to show how the internal stresses in a coiled conventional hollow waveguide affect the overall lifetime of the waveguide.
The prior art devices rely on drawn glass tubes, capillary tubes, rolled tubes, parallel metal plates, plastic tubes, or other extruded shapes to co-locate electromagnetic radiation and a gas/vapor or liquid over a pre-determined path length thereby causing the analyte of interest to interact with the light to detect and quantify the analyte(s) present. Prior art devices with high performance (e.g., sensitivity) tend to be large in size and expensive. Hollow waveguides of circular geometry made of metal, glass, or plastic tubing and having diameters of 1 millimeter or less and lengths ranging from 1-4 meters (for high sensitive applications) are common today; those made of silica are commercially available and have been demonstrated to have the lowest losses compared to other prior art examples. Prior art devices can be made to be small but at the expense of sensitivity (e.g., bending losses) and robustness (e.g., internal bending stresses on materials and optical coatings). Prior art devices are dependent on wet-chemistry for internal coatings, which limits the variety of the internal coating options. Prior art devices are therefore not appropriate solutions in applications where small size, high sensitivity, low cost, integration with other sensor components or peripheral components, and long-term robustness are required.